The term collection agency is usually applied to third-party agencies, called such because they were not a party to the original contract. The creditor assigns accounts directly to such an agency on a contingency-fee basis, which usually initially costs nothing to the creditor or merchant, except for the cost of communications. This however is dependent on the individual service level agreement (SLA) that exists between the creditor and the collection agency. The agency will then take a percentage of the debt that is successfully collected; sometimes known in the industry as the "Pot Fee" or potential fee upon successful collection. This does not necessarily have to be upon collection of the full balance and very often this fee is paid by the creditor if they cancel collection efforts before the debt is collected. The collection agency makes money only if money is collected from the debtor (often known as a "No Collection - No Fee" basis). Depending on the type of debt the fee ranges from 10% to 50%(though more typically the fee is 15% to 35%).

Some collection agencies offer a flat fee, typically $10.00, "pre-collection" or "soft collection" service. The service sends a series of increasingly urgent letters, usually ten days apart, instructing debtors to pay the amount owed directly to the creditor or risk a collection action and negative credit report. Depending on the terms of the SLA, these accounts may revert to "hard collection" status at the agency's regular rates if the debtor does not respond.

In the United States, consumer third-party agencies are subject to the Fair Debt Collection Practices Act of 1977 (FDCPA). This federal law is administered by the Federal Trade Commission or FTC. This act limits the hours during which the agency may call the consumer. It also prohibits false, deceptive or misleading representations, and prohibits the agency from making threats of actions the agency cannot lawfully or does not intend to take.

In Great Britain although no similar law exists, all third party collection agencies must hold a consumer credit license under the Consumer Credit Act 1974 in order to carry out their business. Licenses are issued and regulated by the Office of Fair Trading a government body which protects consumers from unscrupulous traders. In order to retain their license third party agencies must work within the framework outlined within the 2003 fair debt collection guidance.